Effective form of organic selenium
The traditional way of supplying organic selenium is in the form of selenised yeast, a product which has been grown in a high selenium medium. Part of the methionine in the yeast protein is replaced by L-selenomethionine. However, the full replacement of methionine by selenomethionine is not possible (Schrauzer, 2006). Yeast contains selenium mainly as selenomethionine but also has a significant amount of selenium in other forms (Burk, 2006). In the intestinal tract of the animal, the protein in selenised yeast is broken down into small peptides and free amino acids. The selenomethionine in its free form can be absorbed in the intestine as an amino acid in the same way as methionine (Figure 1). In the metabolism, selenomethionine enters the methionine pool and can be incorporated into body proteins. Via this way selenomethionine is able to build up selenium reserves in the body. However, all other forms of selenium in selenised yeast follow the same pathway as inorganic selenium. They are reduced to generate hydrogen selenide in the selenium pool, which in turn is converted to selenophosphate for selenoprotein biosynthesis or is excreted. Therefore the remaining part of selenium in selenised yeast is not considered to be more effective than inorganic selenium. Only the L-selenomethionine in selenised yeast is the effective organic selenium (Figure 2). Several producers have registered selenium yeast products for the animal nutrition market. In Europe, the minimum level of selenomethionine in these products is set at 63%. If we take into account the digestibility of selenised yeast (estimated 80%), it can be concluded that the amount of effective digestible organic selenium is around 50% (63% selenomethionine x 80% digestibility). In practice the concentration of selenomethionine in selenised yeast is widely variable and even the minimum of 63% is not always achieved. This has been demonstrated in a review of in total 11 commercial products, where an average of 51.7% was found with a range of 24.8-69.7% (AllAboutFeed, August 2012).